Observatory build should try backup method if the SDK doesn't work.

tools/build.py

Index: tools/build.py
diff --git a/tools/build.py b/tools/build.py
index fc2e1cfa14923e15daed772e854545237d4a5ecf..0c5e235c16d4b80dada3bf751bb303ae4fb2904c 100755
--- a/tools/build.py
+++ b/tools/build.py
@@ -77,11 +77,6 @@ def BuildOptions():
       help='Name of the devenv.com/msbuild executable on Windows (varies for '
            'different versions of Visual Studio)',
       default=vs_executable)
-  result.add_option("--use-bootstrap-for-observatory",
-      help='Use a stripped down Dart binary built on the host machine '
-           'for building Observatory. Necessary on Linux machines which have '
-           'libc incompatibilities with the prebuilt Dart binaries.',
-      default=False, action="store_true")
   return result
 
 
@@ -391,8 +386,6 @@ def BuildOneConfig(options, target, target_os, mode, arch, override_tools):
   global filter_xcodebuild_output
   start_time = time.time()
   os.environ['DART_BUILD_MODE'] = mode
-  if options.use_bootstrap_for_observatory != False:
-    os.environ['DART_USE_BOOTSTRAP_BIN'] = '1'
   build_config = utils.GetBuildConf(mode, arch, target_os)
   if HOST_OS == 'macos':
     filter_xcodebuild_output = True